For the absorption of various vibrations and impacts exerted by a road surface to a traveling automobile, suspensions furnished with a buffer action are interposed between the automobile body and the axles. These suspensions fulfil important functions of supporting the automobile on the road surface, transmitting the propulsive force from drive wheels to the automobile body and, at the same time, moderating the impacts from the road surface and protecting the automobile against breakage, and further improving comfortableness of ride and stability of operation. With the growing trend of automobiles toward higher speeds, these functions have come to be viewed as factors of increasing significance capable of determining the limits to speed increase. Scientific studies are being promoted comprehensively on the system and have been developing numerous mechanisms and component elements as a consequence.
The suspensions are generally required to be pliable in the vertical direction and rigid in the longitudinal and lateral directions. From the structural point of view, they are broadly divided under two categories, i.e. the axle suspensions and the independent suspensions. The axle suspensions are generally used in front wheels and rear wheels of trucks and in rear wheels of passanger cars. In contrast, the independent suspensions are predominantly used in front wheels and rear wheels of passenger cars which attach primary importance to comfortableness of ride and stability of operation.
The independent suspensions are designed to allow laterally opposite wheels freedom of independent motion instead of requiring them to be interconnected through the medium of one axle. From the structural point of view, they may be broadly divided into Wishbone type, MacFarson type, trailing arm type, and swing axle type. These independent suspensions, as compared with the axle suspensions, have the advantage that when either of the laterally opposite wheels runs on an object projecting from the road surface, the independent suspension serving the particular wheel acts like the knee joint of man, enabling the wheel exclusively to jog vertically thereby preventing the automobile body from a tilt and allowing the automobile to operate stably by effectively avoiding the phenomenon of rolling.
In the various types of independent suspensions, the Wishbone type suspensions are used most widely as mentioned in Japanese Utility Model Application Disclosure SHO 53(1978)-26,020. The independent suspensions of this type are characterized by the fact that since the link mechanisms using two arms produce a parallelepipedal action, the wheels served by the suspensions move substantially vertically and the tires mounted thereon, therefore, contact horizontally the road surface at all times and enjoy a highly satisfactory road surface contact property. They are handicapped by weight and cost because of their complexity in structure as compared with the MacFarson type suspensions. Moreover, they have room for further improvement in respect that the two arms used in each suspension go to decreasing the inner volume of an engine room because they are adapted to thrust into the engine room. The advantage of the Wishbone type independent suspensions that they are robust structurally and excellent in operational stability manifested as during the cornering has come to attract attention again in recent years.
In order that the automobile bodies furnished with such Wishbone type suspensions may fulfil such requirements as alleviation of frictional wear of tires, ability to control the operation of steering, and prevention of transmission of vibrations to the steering handle, the lengths of an upper arm and a lower arm, the positions of fixation thereof (the distance separating them), etc. must be suitably selected.
The lengths of the upper arm and the lower arm and the positions of their fixation (the distance separating them), however, are restricted to a large extent by the spaces allocated to the layout of the upper arm and the lower arm in an automobile on which the suspensions are actually mounted. For the purpose of improving the stability of operation during a gyration, for example, the idea of disposing control arms in such a manner that the camber relative to the road decreases to 0(zero) degree even during the gyration may be conceived. It is, therefore, desirable to give the upper arm a suitable length such that the angle of vibration of the upper arm due to the vertical motion of the corresponding wheel will conform to the angle of vibration of the automobile body during a gyration. An addition to the length of the upper arm, however, results in inevitable protrusion into the engine room of a fulcrum serving to support pivotally the upper arm. Since the effort directed to improving the operational stability brings about serious adverse effects on the design of automobile style, the design of engine, etc., the conventional suspensions have been compelled to sacrifice their performance to a certain extent.
Further, in the conventional suspensions, the layout of the upper arm and the lower arm as viewed from the front side of the automobile body is univocally decided by the characteristic of variation in roll center height, the characteristic of variation in scuff, and the characteristic of variation in camber and the range of selection of the magnitude of each of these characteristics is extremely restricted by the magnitudes of the other two characteristics. The conventional suspensions, therefore, are destitute of freedom of design. A study of the layout of the upper arm and the lower arm whose characteristic of variation in camber is such as to give 0(zero) degree as the angle of camber relative to the road, for example, reveals that this layout degrades the operational stability during a gyration because it results in an undue increase of the variation in scuff and consequently aggravates the lateral vibration of the automobile body or the displacement of the roll center.
Further, it is desirable as described above that the camber angle of the wheels should vary, during a gyration, in the direction of negative camber proportionately to the angle of vibration of the automobile body so as to reduce the angle of camber to 0(zero) degree relative to the road. When any of the wheels, during a straight travel of the automobile, runs on an object projected from the road surface and generates a bump stroke exceeding a stated limit, the wheel gives rise to a camber thrust. It may well be concluded, therefore, that the control of the inclination toward negative camber is desirably started at the time of occurrence of this camber thrust for the sake of improving the operational stability during the straight travel. Since the conventional suspensions show a large inclination toward negative camber in proportion as the vertical stroke of wheel increases, they have never been able to satisfy simultaneously the operational stability during a gyration and the operational stability during a straight travel.
In the independent suspensions of the types disclosed in Japanese Patent Application Disclosure SHO 64(1989)-189710, Japanese Utility Model Application Disclosure SHO 62(1987)-189,904, etc., for example, a link is interposed between an upper arm and a knucle and this link and a lower arm are interconnected through the medium of a connecting link so that the optimum camber characteristic to be required may be attained by causing the link to produce a rotational displacement and impart a lateral displacement to one end part of the knuckle proportionately to the vertical vibrations of the upper arm and the lower arm. The adoption of a link mechanism of this nature, however, is not desirable for the construction of suspensions because this link mechanism exerts a draft in the axial direction on the ball joint disposed in the connecting part of the knuckle.